The present invention relates generally to semiconductor fabrication, and more particularly to optical proximity correction (OPC) photomasks for use in such semiconductor fabrication.
FIG. 1 shows an example photomask 100 having a semiconductor design with a main feature 102, according to the prior art. The main feature 102 is specifically a line, which has a critical dimension CD, which is specifically a width. Preferably, the main feature 102 should be imprinted on a layer of photoresist in a manner such that the critical dimension CD does not change. However, due to diffraction or other causes, the main feature 102 is actually imprinted on the photoresist with a width that is less than the desired critical dimension CD.
FIG. 2 shows an example in which the photomask 100 of FIG. 1 is used within a lithographic process to imprint the main feature 102 on a layer of photoresist 156 that has been coated over a semiconductor substrate 154, according to the prior art. The photomask 100 is made up of a transparent substrate 152, such as quartz, and an opaque layer 164, such as chrome, in which the main feature 102 having the critical dimension CD has been patterned. The photoresist 156 is exposed to light 160 or another source of radiation through the photomask 100. Preferably, the non-exposed area 162 within the photoresist 156 should have a critical dimension, or width, 158 that is equal to the critical dimension CD of the main feature 102 within the mask 100. However, due to diffraction or other causes, as has been noted the actual width 158 is less than the critical dimension CD of the main feature 102, which is undesirable.
OPC can be used to correct the isolated-dense proximity effect and the isolated-feature depth of focus reduction by adding assist features known as scattering bars (SB's) and anti-scattering bars (ASB's) near the edges of opaque and clear features, respectively, on a photomask. SB's are sub-resolution opaque-like features, whereas ASB's are sub-resolution clear-like features. Both serve to alter the images of isolated and semi-isolated lines to match those of densely nested lines, and improve depth of focus so that isolated lines can be focused as well as dense lines with the lithography equipment.
One of the goals in integrated circuit fabrication is to faithfully reproduce the original circuit design on the semiconductor substrate (via the mask). Currently, various optical proximity correction (OPC) techniques are utilized to allow the resulting image to more accurately correspond to the desired target pattern. A common OPC technique, which is widely known, is the use of subresolution scattering bars (also referred to as assist features). For example, U.S. Pat. No. 5,821,014, discloses sub-resolution assist features, or scattering bars, used as a means to correct for optical proximity effects and have been shown to be effective for increasing the overall process window. As set forth in the cited patent, generally speaking, the optical proximity correction occurs by improving the depth of focus for the less dense to isolated features by placing scattering bars near these features. The scattering bars function to change the effective pattern density (of the isolated or less dense features) to be more dense, thereby negating the undesirable proximity effects associated with printing of isolated or less dense features. It is important, however, that the scattering bars themselves do not print on the wafer. Thus, the size of the scattering bars must be maintained below the resolution capability of the imaging system.
Notwithstanding the wide-spread use of scattering bars, there remain essentially three issues with current scattering bar technology when utilized for patterning feature dimensions at half or below the exposure wavelength. The first issue relates to inadequate protection for the main design features that severely limits focus range. The second issue relates to the fact that in a typical scattering bar solution, too many short pieces of scattering bars are generated which results in excessive demands on mask making capabilities. The third issue relates to the fact that there is no adequate solution for joining adjacent horizontal and vertical scattering bars together.
It is crucial for the assist features to be sub-resolution and have critical dimensions less than those of their corresponding main features, to ensure that the assist features do not imprint on photoresist during photolithography. Where more than one such photomask is needed, process control also becomes an issue, as it must be ensured that all the photomasks have assist features with critical dimensions less than their corresponding main features. Mask defect inspection also becomes more difficult to verify that the assist features have critical dimensions less than their corresponding main features.
Therefore, there is a need for an improved OPC mask that alleviates these shortcomings. In particular, there is a need for such a mask in which the assist features can be more easily added, but where the assist features will not be imprinted on the photoresist during photolithography. For these and other reasons, there is a need for the present invention.